We have continued our studies on the purification and regulation of ornithine decarboxylase of S. cerevisiae, the rate-limiting enzyme in the biosynthesis of putrescine, spermidine, and spermine. We have prepared an antibody column, which putrescine, spermidine, and spermine. We have prepared an antibody column, which permits us to obtain 705 pure enzyme in one step. We have also developed immunological methods that permit us to study an enzyme in a crude fresh extract. We have now found evidence that the enzyme in vivo differs in size (80,000 MW) from the enzyme which we purified by standard techniques (68,000 MS). The changes result from proteolysis, since addition of a proteolytic inhibitor markedly slows the conversion. Strains with high enzyme activity, our mutants (spe10) lacking ornithine decarboxylase, cells which have lost their ornithine decarboxylase activity as a result of growth with amines, and wild type cells with very low activity, all have equal enzyme protein bands in the 86,000 position by our techniques. Thus post-translational modification must account for changes in enzyme activity.